1. Field of the Invention
The present invention generally relates to physical vapor deposition. More particularly, the invention relates to a copper sputtering target for use in a physical vapor deposition chamber and process.
2. Background of the Related Art
As circuit densities increase for the next generation of ultra large scale integration, copper is becoming a choice metal for filling nanometer-sized, high aspect ratio interconnect features on substrates because copper and its alloys have lower resistivities than aluminum and significantly higher electromigration resistance as compared to aluminum. These characteristics are important for supporting the higher current densities experienced at high levels of integration and increased device speed. Copper also provides good thermal conductivity and is readily available in a highly pure state.
FIG. 1 is a simplified cross sectional view of a typical physical vapor deposition chamber 100 useful for deposition of copper. The PVD chamber 100 generally includes a housing 101, a substrate support assembly 102, a cover ring 104, a plasma shield 106, a dark space shield 108 and a target 112. As shown, the PVD chamber 100 may also include a collimator 110 to enhance deposition into apertures on the surface of the substrate. During processing, a substrate 122 is positioned on the substrate support assembly 102, and a processing gas, such as argon, is introduced into a region 120 defined by the target 112, the dark space shield 108 and the collimator 110. A plasma of the processing gas is struck in the region 120 by applying a bias (RF or DC) between the target 112 and the substrate support assembly 102. Ions from the plasma are attracted to and collide with the target to cause sputtering of the deposition material from the target 112 onto the substrate 122.
FIG. 2 is a cross sectional detail of a dark space shield 108 and a target 112 of FIG. 1. The target 112 generally comprises a backing plate 114 typically made of aluminum and a sputterable layer 116 made of a sputterable deposition material such as copper. The backing plate 114 includes a target support flange 124 that rests on (or is otherwise secured on) an insulator ring 126 disposed at a top portion of the housing 101. The insulator ring 126 provides electrical insulation between the target 112, which is typically biased at a particular voltage, from the housing 101, which is typically grounded. An O-ring 128 disposed in an annular recess 130 on a lower surface of the target support flange 124 provides a seal between the target support flange 124 and the insulator ring 126 to enable a vacuum environment to be maintained in the chamber 100. Typically, a small gap 132 is formed between the lower surface 138 of the target support flange 124 and an upper surface 140 of the insulator ring 126.
The dark space shield 108 is secured on the housing 101 and supports the collimator 110 at a lower end of the dark space shield 108 (as shown in FIG. 1). An upper end of the dark space shield 108 is disposed between the insulator ring 126 and the sputterable layer 116 of the target 112. A dark space gap 134 is formed between an upper surface 136 of the dark space shield 108 and the lower surface 138 of the target support flange 124, and an entry gap 142 is formed between an annular curved edge 144 of the sputterable layer 116 and an inner surface 146 of the dark space shield 108. The annular curved edge 144 of the sputterable layer 116 creates an overhang portion 148 that narrows the entry gap 142. The backing plate 114 includes a recessed comer portion 150.
Although a variety of advanced PVD techniques, including long throw, collimation and ionized metal plasma, have been successfully demonstrated for copper deposition by PVD (PVD Cu), a number of obstacles are presented for PVD Cu because of the properties and characteristics of copper such as a higher melting point, a lower sticking coefficient, relative inertness and higher sputtering yields as compared to other metals, such as aluminum and titanium. PVD Cu generally deposits in an over-cosine deposition pattern with less directionality in which the sputtered Cu atoms mostly do not follow a line of sight deposition and may bounce off a number of surfaces before becoming bonded to a final surface. Additionally, sputtered Cu atoms form relatively weak bonds on most surfaces and can be detached by other impinging energetic Ar/Cu ions or atoms. Currently, one of the preferred PVD Cu techniques is a Low-Pressure Long-Throw (LPLT) process wherein the pressure of the chamber during processing is maintained between about 0.2 mTorr and about 1.0 mTorr and the spacing between the target and the substrate is typically between about 100 mm and about 300 mm. The properties and characteristics of copper and PVD Cu present a number of problems in the LPLT PVD Cu chamber that were not encountered in other PVD Cu techniques and deposition of other metals.
First, plasma is not restricted by the overhanging portion 148 from entering through the entry gap 142 and reaching surfaces above the overhanging portion 148 because the sputterable layer 116 is not thick enough to create an entry gap 142 that is long enough to prevent entry of the plasma. Plasma that reaches the surfaces beyond the overhanging portion 148 causes sputtering of the target material, including both the sputterable material and the backing plate material, from the surfaces between the overhanging portion 148 and the recessed corner portion 150. Furthermore, this problem is exacerbated in a low pressure environment because copper can self-sustain plasma in a sub-mTorr environment after plasma ignition due to the lower requirement of an electron escaping energy from the target.
As shown in FIG. 2, the sputtered material from the surfaces above the overhanging portion 148 has a direct line of entry into the dark space gap 134. Material sputtered from the overhanging portion 148 can directly enter the dark space gap 134. The sputtered particles that enter the dark space gap 134 can reach the small gap 132 and deposit onto the surfaces in the small gap 132 and the O-ring 128. The sputtered particles that enter the dark space gap 134 can also deposit onto the exposed surfaces of the insulator ring 126. The unwanted deposition on the O-ring 128 and the insulator ring 126 becomes a particle source that may generate particles that contaminate subsequently processed substrates. The unwanted deposition on the O-ring 128 and the insulator ring 126 also causes cross-contamination problems with other non-copper components. The deposition on the O-ring 128 and the insulator ring 126 may also cause an electrical short or arcing between the target 112 and the dark space shield 108 that may damage both the target 112 and the dark space shield 108.
Secondly, in a low pressure environment, the sputtered copper atoms have a mean free path that allows the copper atoms to deposit onto surfaces that were previously unreachable in higher pressure PVD Cu processes. Because of a decrease in the number of argon atoms in the low pressure plasma environment, the sputtered copper atoms travel longer distances before colliding with argon atoms. Additionally, the sputtered copper particles tend to bounce off a number of surfaces before sticking to a final surface because the sputtered copper has a low sticking coefficient. The combination of a long mean free path and a low sticking coefficient allows the sputtered copper particles to deposit onto surfaces that were previously unreachable and uncontaminated in other PVD Cu processes, such as the surfaces within the gap 132 and the O-ring 128. As discussed above, the deposition onto these surfaces may cause damage to the target 112, the dark space shield 108 and/or other subsequently processed substrates.
Another factor that contributes to deposition onto these surfaces that were previously uncontaminated is that the LPLT sputtering technique requires a longer process time because LPLT sputtering of copper has a lower deposition efficiency as compared to other PVD Cu techniques. The longer processing time leads to an increased temperature during processing that increases the mobility of the copper atoms and contributes to deposition of copper onto surfaces that were previously inaccessible to the sputtered copper atoms.
The deposition on the surfaces within the gap 132, the O-ring 128 and the insulator ring 126 becomes a particle source that may generate particles that contaminate subsequently processed substrates. Particularly with copper deposition, the copper contaminant particles may cause cross contamination problems with aluminum based front end devices. Also, the deposition on the O-ring 128 and the insulator ring 126 may cause an electrical short or arcing between the target 112 and the dark space shield 108, which may result in improper processing and defect formation on the substrate. Since the O-ring 128 and the insulator ring 126 are typically part of a processing kit that is replaced periodically, the deposition of copper onto these surfaces requires more frequent replacements of the process kit and reduces throughput because of the excess time spent removing and reattaching the process kit.
Therefore, there remains a need for a target for a PVD chamber for deposition of copper that significantly reduces deposition on the surfaces within the target dark space and the insulator ring and prevents deposition on the O-ring to reduce contaminant particle generation and to prolong the useful lifetime of the process kit.